Known domed piston/piston-rod assemblies such as those used in reciprocating hot gas engines utilize a unitary piston-rod/crosshead member assembled to a multi-member piston assembly including a base and a dome. A tapered portion of the piston-rod/crosshead is frictionally retained in a tapered joint in the base member of a piston assembly and a locking nut further secures the rod to the base. The dome is then attached to the base, usually by a threaded attachment. A detachable dome element is necessary to allow access to the locking nut.
Controlled radial and axial clearances are critical for good performance in all reciprocating machinery. In hot gas engines, the concentricity or radial alignment of the piston and piston-rod determines the wear and leakage of the seal elements required between the working fluid in the cylinders and the lubricant in the crankcase. Leakage reduces engine performance. The angular alignment of the piston and rod can cause a radial displacement of the piston top from its ideal position. This is significant in a hot gas engine due to the required dome length. To avoid rubbing contact between a poorly aligned piston dome and cylinder, a larger gap clearance is required, which reduces engine performance.
In operation, the combination of tapered and threaded joints of known assemblies results in poor tolerance control. The tapered joint is intended to provide good angular and concentric alignment between the rod and the piston. However, it creates poorly defined axial length. The threaded attachment of the dome results in poor angular and concentric alignment and so nullifies the good radial alignment of the tapered joint. Furthermore, the assembly is needlessly heavy due to the multiple joints, complicating engine balance. Additionally, working fluid may leak into the two piece piston assembly since it is difficult to obtain a perfect seal at the threaded attachment of the dome and base.